The present invention relates to a method of and apparatus for removing liquids from solids or non-liquids. More specifically, the present invention relates to a method and apparatus for dewatering expanded or flocked materials or any other solid, in particulate or other form, having a large surface area which holds substantial quantities of water thereon or therein. The present invention may be used to remove water from natural and synthetic polymers or elastomers such as flocked latexes, crumb rubber, other rubber materials, and plastic materials, particularly polystyrene foam and similar materials. The invention may also be used to dewater fibrous or pulpy material such as wood or cane.
Many manufacturing and use activities involving polymers result in large amounts of water-containing material from which the water must be removed. For example, in making rubber a solution or slurry of rubber in a solvent may be produced. Steam and water are percolated through the slurry to vaporize the solvent, leaving behind a rubber particulate having a high water content. This water must be removed before the rubber can be conveniently processed into articles or products.
Another area of concern relates to those materials which result from fast food and other restaurant operations in which food and drink are served in and on a variety of plastic media, such as plates, cups and compartmentalized carriers. These materials become waste after the comestibles formerly borne thereby has been consumed.
The disposition of this plastic waste raises at least two concerns. First, the plastic waste is typically not rapidly biodegradable. Thus, disposal thereof by conventional procedures, such as those utilizing land fills, may be objectionable to some. Burning the waste may be potentially objectionable if the combustion products are noxious or are health threatening. Second, the costs of producing the materials, and the materials themselves, would be irrevocably lost if the materials were simply disposed of. Many plastic materials can be re-used to manufacture subsequent plastic products if a cost-effective way is found for placing them in a condition suitable for such re-use.
The use of polystyrene materials as dishes results in their being contaminated with food and drink. To re-use the materials, they are subjected to particulation or comminultion and to washing or rinsing with water in an effort to remove the food and drink contamination therefrom. Such washing or rinsing adds substantial quantities of water to the particulate materials.
It is well known that plastic materials such as polystyrene foam can be reused and that such re-use is best achieved with the materials comminuted and "dry", that is, in a particulate, granular or flake form with much of the water removed therefrom. The comminuted plastic is typically transported and worked in a continuous screw press or barrel-screw apparatus, such as an extruder, at the output of which the material is extruded. The extrudate is cut to produce pellets of the material. These pellets are re-used to produce foamed polystyrene products.
If the comminuted plastic is not dry, the working thereof by an extruder is inefficient owing to the fact that the majority of the heat generated by working the plastic is initially absorbed by the water. This renders the working process process inefficient. It also requires that vents or pressure relief facilities be present on the extruder so that substantial quantities of steam resulting from heating the water do not cause substantial steam evolution and/or violent escape thereof which can render extrusion inefficient or ineffective. In any event, it is required that steps be taken or facilities be provided to safely handle large quantities of steam. If an extruder could be designed to accommodate the presence of substantial quantities of water in the comminuted material, the transport and working thereof would proceed quite slowly, since heating and driving off water--having a high specific heat--requires time and energy. Water removal achieved by heat input is expensive and slow.
The prior art contains various aparatus for dewatering elastomers or polymers. U.S. Pat. No. 3,672,641 discloses apparatus for removing liquids from polymers which includes an extruder. The barrel of the extruder includes plural screen bars which define therebetween drain openings communicating with the barrel bore. Expansion chambers--increased diametric sections of the barrel bore--are preceded by restricted passages defined by collars on the extruder screw and a closely spaced choke on the bore wall. Liquid is said to be pressed from material worked by the extruder, the liquid exiting the barrel via the drain openings. A high degree of working is also said to "superheat" the water in the worked material so that the water in the material-water combination vaporizes and is "flashed off" in the lower pressure expansion chambers via the drain openings. Experience has shown that the material tends to clog or block the drain openings, necessitating continuous cleaning thereof. Further, the steam which is intentionally produced must be safely handled and conducted away. The apparatus of U.S. Pat. No. 3,587,450 is similar to that of the '641 patent.
In U.S. Pat. No. 3,035,306 a continuous screw press includes a longitudinally grooved barrel and a screw, the flight of which is spaced from the screw stem. Water-containing material is worked by the press expressing water therefrom which is said to reversely flow through the grooves and the flight-stem spaces. This reverse flow is counter to the direction of material movement and causes the water to move to a point up stream of material feed, whereat it drains from the barrel. The material tends to clog the drain as well as the grooves and the flight-stem spaces. Such clogging and the movement of the material tend to cause expressed water to be trapped downstream of the feed to the detriment of the efficiency of the process and the quality of the material. The dewatering portion of the aparatus in U.S. Pat. No. 4,490,104; 4,493,630; 2,994,105; 3,768,171; and 3,595,162 are similar in function to that of the '306 patent.
In view of the foregoing, the present invention contemplates a method and apparatus for dewatering materials, including waste plastics, which avoid the problems summarized above and permit the cost-effective processing and use of such materials.